Material crushing and grinding means and method



Nov. 29, 1960 J. F. MEISSNER 2,962,229

MATERIAL CRUSHING AND GRINDING MEANS AND METHOD Filed 001;. 20, 1958 2 Sheets-Sheet 1 l 55; 1/ 25 E q F/MJf/ED F//Vj9 f; J0 24 25 16' 22 21 3 C! i I) 27 Pg 0 0 Raf-20 //Vf/?MD/4 12 13 17 14/ A3125 L 13 Z. 84 26 2g Kg 81 IN V EN TOR.

Nov. 29, 1960 J. F. MEISSNER MATERIAL CRUSHING AND GRINDING MEANS AND METHOD Filed Oct. 20, 195B 2 Sheets-Sheet 2 United States Patent 0 MATERIAL CRUSHING AND GRINDING MEANS AND METHOD John F. Meissner, La Grange, Ill. (300 W. Washington St., Chicago 6, Ill.)

Filed Oct. 20, 1958, Ser. No. 768,298

7 Claims. (Cl. 241-5) This invention relates to means for and a method of crushing and grinding materials, such as ores, rock, coal and various other materials of a character to be broken up or crushed by impact.

It is known to crush and to reduce various ores and analogous materials to a finely divided or powdered condition by successive crushing steps in crushers followed by successive grinding steps in ball mills, rod mills, or other suitable apparatus. The known methods referred to involve the use of expensive apparatus which requires large amounts of power for its operation and is subject to rapid wear by abrasion often requiring extensive repair or replacement. It also is known to reduce such materials by dropping pieces or lumps thereof onto a succession of breaker plates thereby breaking the larger pieces into smaller pieces, by impact with the breaker plates, and incident thereto, producing to limited extent, fine particles of the material in more or less powdered form. The latter method eliminates the expensive crushing and grinding apparatus of the presently known and used method first above referred to, but it is essentially a breaking or crushing method with no substantial grinding of the material under treatment, so that the amount of material produced in fine particle or powdered form is comparatively small.

I have discovered that by subjecting to the impact of heavy pieces or lumps of the material under treatment, falling freely from a substantial height, a bed of smaller pieces of material and of substantial thickness or depth, it is possible to break up or crush the large heavy pieces and, by the gravity impact thereof with the bed, to impart to the material of the bed a combined high pressure crushing and grinding action effective for rapidly reducing the material of the bed to small size and to fine particle or powdered form in large volume and at a rate greater than that of either the present method employing successive crushing and grinding steps in crushing and grinding mills, or the method in which pieces of material are dropped onto breaker plates, above mentioned. In practicing the method of my invention I provide a tower of substantial height and unobstructed interiorly, from the top of which large or heavy pieces or lumps of the mate rial to be reduced fall freely onto a bed of smaller pieces of the material and which is of substantial depth. The impact of the heavy pieces with the bed crushes the smaller pieces and also causes them to grind together under heavy pressure effective for rapidly reducing them to fine particles or powdered form. Preferably, the bed of material is advanced across the tower at a predetermined rate for discharge therefrom of the crushed and ground material of the bed and of the larger pieces or lumps of material on top of the bed. The material, including the larger pieces or lumps, discharged from the bed is graded by suitable means, conveniently by screening, the larger pieces or lumps are returned to the top of the tower and again dropped on the bed, the smaller pieces are returned to the bed at a rate to maintain the desired predetermined thickness or depth thereof, and the fines may be delivered to a storage bin or to any suitable point. The extra fine certain cases.

particles of material, which may be considered as in powdered form, conveniently are drawn off from the tower by suction means and delivered to dust collectors from which the powdered material may be delivered to a storage bin or any suitable point. The method of my invention is a continuous one in which the material is recirculated through the crushing and grinding means, supplemented by large or heavy initial feed lumps of material delivered to the top of the tower, until it has been reduced to the desired degree of smallness or fineness of particle size. The crushing and grinding means for practicing the method of my invention is continuous in operation, possesses high capacity and is less expensive than the presently used crushing and grinding mills of equal capacity. Further, the power required to operate the apparatus of my invention is substantially less than that required to operate the primary, secondary and tertiary crushers and the grinding mills, frequently necessary under the present practice first above referred to, to reduce certain materials to the required degree of fineness of particle size.

The effective height of the tower and the depth of the bed of material may vary in accordance with the character of the material under treatment. Taconite, for eX- ample, is high siliceous and does not readily fracture. On the other hand, certain other iron ores, from Canada and New York, have a high lime content and readily fracture. In the case of materials difficult to fracture, the heavy pieces or lumps should be dropped from a greater height than those of more readily fractured pieces, and the bed should be of somewhat greater depth in the former case than in the latter case. In general, the effective height of the tower may be from approximately thirty to seventy feet, more or less, and the depth of the bed of material may be from approximately eight to ten inches, more or less. The essential requirement is that the height from which the large or heavy pieces or lumps of material are dropped is such as to assure fracture or breaking thereof, and the depth of the bed is such as to assure substantial grinding of the material by the grinding together of the pieces of the bed material under the gravity impact of the free falling heavy pieces of material.

In order to obtain maximum production of fines, including extra fine or powdered material, the bed of crushed and ground material should be maintained at maximum depth, so that the large or heavy lumps of material will penetrate deeply into the bed to effect rapid grinding of the material thereof by attrition or rubbing together of the pieces of material of the bed under the high impact pressure of the larger or heavy lumps or pieces of material. The cushioning effect of the thick or deep bed of material reduces the shattering effect on the large pieces of material, so that by recirculating those pieces with the initial feed of large pieces, assurance is had that the bed is subjected to substantially continuous pounding by large or heavy pieces of the material under treatment.

It is not essential to the method of my invention that the material be reduced to powdered condition. It may be reduced to any extent desired and the material crushed or ground to the desired size or degree of fineness may be recovered by screening or in any suitable manner, so as to recover aggregate of coarse or intermediate size while also producing fines in large volume. If the material is ground to powdered form it may be withdrawn from the tower by suction, as above indicated, or may be recovered in any suitable manner. In either case, I preferably take off from the tower the finely powdered material or dust generated therein, though that is not essential to the broader aspects of my invention and may be omitted in Further objects and advantages of my invention will appear from the detail descriptions.

In the drawings:

Figure l is a semi-diagrammatic side elevational view of material crushing and grinding means embodying my invention, with the upper portion of the tower shown in section;

Figure 2 is a plan view, on an enlarged scale, of the means of Figure l, certain parts being omitted and certain other parts being in part broken away;

Figure 3 is a sectional view, on an enlarged scale, taken substantially on line 33 of Figure 2, the tower being broken away for a substantial portion of its height and certain parts being omitted for clearness of illustration;

Figure 4 is a sectional view taken substantially on line 44 of Figure 3;

Figure 5 is a sectional view, on an enlarged scale taken substantially on line 5-5 of Figure 4;

Figure 6 is a sectional view taken substantially on line 66 of Figure 5;

Figure 7 is a top plan view, on an enlarged scale, of one of the bars of the bed conveyor; and

Figure 8 is an underneath view of the bar of Figure 7.

The crushing and grinding means of my invention comprises a tower 10 of substantial height, as above indicated, and of suitable construction, to be described more fully later. The material to be treated is delivered to the top of the tower 10 by an endless conveyor 11, which is the main feed conveyor and may be of any suitable conventional type driven in a known manner. The material is delivered to conveyor 11, in large or heavy pieces or lumps, in any suitable manner. Conveniently, a ramp 12 is provided leading to a feed hopper l3 mounted in the pit l4 and having an associated delivery conveyor 15 which discharges onto the lower end portion of the feed conveyor 11. The ramp 12 provides means whereby suitable vehicles, such as a dump truck 16, may be positioned above and adjacent the hopper 13 for discharge of the material into the latter, as indicated in Figure l. A recirculating conveyor 17, which may be of conventional type driven in a known manner, delivers material from the bottom of tower 10 to suitable screens, also of conventional type, enclosed in a housing 18. Three spouts 19, and 21 lead from the screen housing, spout l9 delivering the fines to a fines conveyor 22, spout 20 delivering the large pieces or lumps of material to the feed conveyor 11 and spout 21 delivering the smaller broken pieces of material to a return conveyor 23 which discharges into tower 10 a short distance above the bottom thereof. The conveyors 22 and 23 also are of conventional type driven in a known manner. A takeoff manifold 24 is connected by branches 25 thereof to tower 10 at different levels thereof and opens, at its lower end, into a housing 26 enclosing a motor-driven fan (not shown) eflective for withdrawing dust-laden air from tower 10, through manifold 24 and branches 25 thereof. The dust-laden air is discharged from the housing 26, through a conduit 27, to dust collectors 28 of suitable known type, from which the collected dust is delivered to an endless conveyor 29, of conventional type driven in a known manner, which delivers the dust to a storage bin or to any suitable point. The fines conveyor 22 delivers the fines, which it receives from the spout 19 of the screen assembly, to a storage bin or any suitable point. The tower 10 may be of any suitable construction and cross section. Conveniently it is of rectangular cross section and is formed of steel plates 32 and 33 secured to upright corner I-beams 34 and angles 35. The plates 32 and 33 preferably are braced, at suitable intervals, by brace members 36 extending between and secured to the corner I-beam uprights 34.

The lower portion 39 of tower 10 is of downwardly converging substantially frusto-pyramidal shape, as shown, and is provided, at its lower end, with a downwardly extending rectangular neck 40 provided with a re-enforcing inner member or liner 41 and open at one side, as shown in Figure 5. A bed conveyor 42 is disposed with its upper run underlying neck 40 in proximity thereto. The conveyor 42 is formed of a plurality of bars 43, shown more clearly in Figures 7 and 8, somewhat similar to grate bars, provided at each side with interengaging fingers 44 and slots 45. The fingers 44 constitute extensions of re-enforcing ribs 46 extending transversely of bar 43 and disposed at the under faces of the bars at the upper run of the conveyor 42. The interengaging or interfitting fingers 44 receive at their end portions pintle rods 47 passed therethrough and hingedly connecting the bars 43 thus providing an endless conveyor. Each of the bars is provided, at each end thereof, with a flange 48 disposed to extend upwardly from the upper run of the conveyor, such flanges being exterior of neck 40 and in proximity to opposite sides thereof. The side edges of the bars 43, which are the leading and following edges thereof in the conveyor, are beveled as shown in Figures 5 and 8, to accommodate travel of the bars about the associated support and drive members while assuring that the flanges 48, at the upper run of the conveyor 42, will be disposed in endwise contact during travel along the opposite sides of neck 40, as shown in Figure 5.

The conveyor 42 is mounted on a supporting framework of heavy construction comprising posts or uprights 50 to the upper ends of which is secured a framework 51 comprising side [-beams 52 connected by cross I- beams 53 welded or otherwise suitably secured to beams 52. I-beams 53 receive l-beams 54 seating thereon and extending transversely of frame 51 and suitably secured thereto, conveniently by welding. A plurality of bearing blocks 55 of heavy construction seat on the upper flanges of the I-beams 54 and are bolted or otherwise suitably secured thereto. A shaft 56 is secured through the hearing blocks 55 of each of the I-beams 54 and rotatably receives, between the bearing blocks 55, rollers 57, the latter and shaft 56 being of heavy construction. The ribs 46 of bars 43 at the upper run of conveyor 42 travel upon the rollers 57 and it is thus supported, at its upper run, by a rugged and shock-resistant structure.

The conveyor 42 passes about two sprocket wheels 59 and 60 each in the form of a drum of hexagonal cross section provided with slots which receive the ribs 46 of bars 43, each face of the drum being of a width to receive one of the bars 43 and each corner of the drum being provided with a groove or recess 61 which receives the portions of the respective pintle rods 47 between the fingers 44 for driving the conveyor. The sprocket wheel 59 is fixed on a shaft 62 rotatably mounted at each end in a block 63 slidable in a bearing housing 64 bolted to I-bear 53 of frame 51. Bearing block 63 is adjustable outward of conveyor 42 by an adjusting bolt 65 threaded in housing 64, in a known manner, to take up slack in conveyor 42 and assure proper engagement thereof with the sprocket wheels. The sprocket wheel 60 is the drive sprocket for conveyor 42 and is secured on a shaft 66 rotatably mounted in bearing blocks 67 suitably secured, as by bolting, to I-beams 53. The shaft 66 is connected by a flexible coupling 68 to the output shaft of a speed reducer 69 the input shaft of which is connected by a flexible coupling 70 to an electric motor 71, preferably a direct current motor, of suitable power. The sprocket wheel 60 is rotated counterclockwise, as viewed in Figure 5, so that the upper run of conveyor 42 is driven toward the left.

Conveyor 42 extends into a housing 73 projecting from the open side of neck 40 of tower 10. An inclined plate 74 extends at an inclination downward from the discharge end of conveyor 42 and defines with the outer wall of housing 73 a chute for directing material discharged from conveyor 42 onto the lower end portion of the return or recirculating conveyor 17, which extends beneath conveyor 42 as shown in Figures 1 and 5. A supplementary conveyor 75, which may be an endless belt conveyor of conventional type, underlies bed conveyor 42 and is supported by members 76 in the form of angle bars secured to and depending from the I-beams 53, the upper run of conveyor 75 being supported by rollers 77 mounted between angle bars 76. Conveyor 75 passes about end rollers 78 and 79, the shaft of the latter roller being extended and having secured thereon a sprocket wheel 80 receiving a sprocket chain 81 which passes about a sprocket wheel 82 secured on shaft 66 of the drive sprocket 60 of conveyor 42. The supplementary conveyor 75 is thus driven in the same direction as the conveyor 42 and receives, on its upper run, any fine particles of material which may pass through conveyor 42. An inclined plate 83 extends downward from beneath the discharge end of conveyor 75 and provides, with plate 74, a chute for directing onto conveyor 17 material discharged from conveyor 75. Preferably, though not necessarily, the lower portion 39 of tower and the associated conveyors and supporting structure are enclosed by a hood 84, shown fragmentarily in Figure 5.

In practicing the method of my invention I provide a bed 86, of substantial depth or thickness, on the upper run of the bed conveyor 42, bed 86 being formed of small pieces of the material under treatment and containing, in substantial portion, finely ground particles of such material produced by grinding together under heavy impact pressure of the pieces of the material of the bed. Having formed the bed on the upper run of the conveyor 42 the latter is driven at such speed as to advance the bed 86 toward the discharge end of conveyor 42 while maintaining the desired depth or thickness of the bed 86. During the advancement or travel of the bed 86 the large or heavy pieces of material are discharged from the feed conveyor 11 at the top of tower 10 and fall freely therethrough onto the top of bed 86, it being noted that tower 10 is free of all interior obstructions and that the material is delivered by conveyor 11 to the tower centrally thereof so as to have a free fall from the upper end of conveyor 11 to the bed 86. The heavy pieces of material strike the bed with high impact effective for shattering or fracturing the large pieces, the impact thereof serving to breakup and reduce the size of the pieces of material of the bed and, in addition, to cause rubbing and grinding together under high impact pressure of the material of the bed, effective for rapidly reducing it to fine particle or powder form. Due to the continuous pounding to which the bed of material is subjected a large amount of material in powdered form is produced and the tower 10 is soon filled with such powdered material, which is taken off through the manifold 24 and delivered to the dust collectors 28 from which the finely powdered material or dust is discharged onto the conveyor 29 and delivered thereby to a storage bin or other suitable point. In the travel of the conveyor 42 the material of the bed 86, together with the larger pieces or lumps of material resting upon the bed, is discharged onto the return conveyor 17, and the finer particles of material which may fall onto the supplementary conveyor 75 are also discharged onto the conveyor 17. The conveyor 17 delivers the material to the screens within the housing 18, and the screens sepa rate the material or grade it according to desired particle size thereof. The fines from the screens are delivered to the fines conveyor 22 and carried thereby to a storage bin or to any other suitable point. The remaining large pieces or lumps of material are returned to the feed conveyor 11 and, together with the original or feed material, in large or heavy pieces or lumps, are discharged from conveyor 11 into the tower l0 and fall therethrough onto the bed of material 86, as before. In that manner the material is recirculated in the crushing and grinding means until it has been reduced to particles of what may be termed intermediate size, which are delivered to the conveyor 23. The intermediate size particles or pieces of material, which are of appropriate size to form the bed 86, are delivered by the conveyor 23 to tower 10 at an intermediate point, which may vary depending upon the nature of the material. In that manner the bed is continuously replenished so as to be maintained at the proper thickness or depth to assure the combined crushing and grinding action due to high impact pressure of the large or heavy pieces or lumps of material falling freely from the top of the tower onto the bed.

The screens within housing 18 may be of any suitable known type and are effective for grading the material delivered thereto by the conveyor 17, as previously noted. Obviously, if desired, the screens may separate out aggregates of two or more sizes, the screen housing 18 being provided with suitably disposed spouts for discharging such aggregates to conveyors similar to conveyor 22 and which deliver the aggregates to bins, stock piles, or other suitable points.

The distance above bed 86 of material at which the conveyor 23 enters the tower 10 may vary considerably depending upon the characteristics of the material under treatment. If the material is of a character to be readily crushed and ground the distance of conveyor 23 above the bed of material 86 may be less than that when the material is highly resistant to crushing and grinding. The conveyor 11 is operated at appropriate speed and the large or heavy lumps or pieces of material discharged from the upper end of that conveyor, which is disposed centrally of the tower 10, fall freely through the central area of the tower onto the bed of material 86, as previously noted. The return conveyor 23 has its discharge end disposed much closer to the corresponding side wall of tower 10 than the discharge conveyor 11, so that the pieces or lumps of material dropping through the tower from conveyor 11 will clear the conveyor 23. The upper end or top of tower 10 is provided with a cover or roof 87, it being understood that the upper portion of the tower provides a penthouse in which the motor for driving the conveyor 11, and associated parts, are disposed.

It will be apparent from what has been said that the method of my invention is a continuous one in which the material under treatment is recirculated until reduced to the desired degree of fineness. The smaller or intermediate particles of material discharged from the return conveyor 23 into the tower 10 are directed by the lower downwardly tapering portion 39 of the tower onto the bed of material 86 effective for maintaining the proper predetermined depth thereof and thereby assure the rapid reduction of the material to the desired particle size, the dust produced being continuously withdrawn from the tower 10 in the manner previously explained. Due to the updraft of air through the tower 10 from the opening therein for the discharge end of conveyor 23 the rate of fall of the intermediate pieces of material from the return conveyor 23 may be impeded to appreciable extent. Accordingly, the height of conveyor 23 above the bed of material 86 should be such as to assure that the smaller pieces of material discharged from conveyor 23 will reach the bed 86 in advance of the heavy pieces or lumps discharged from the conveyor 11. It is also desirable that the distance of conveyor 23 above bed 86 be sufficient to assure that the extra fine particles of material or powder contained in material discharge from conveyor 23 will be, in substantial portion at least, entrained by the air flowing upward through the tower 10 from the opening therein for conveyor 23 so as to be continuously removed therefrom. It will be seen that the method of my invention provides a continuous one in which the material under treatment is crushed and re duced to the desired particle size, and may be reduced to fine particle size or substantially powdered form, quite rapidly by the impact on the bed 86 of the heavy pieces or lumps of material, assuring large volume production, and the means for practicing the method of my invention is highly efficient while being comparatively simple and inexpensive, the power requirements being materially less 7 than that of the grinding and milling machinery required under the present practice such as has been referred to above.

In the above description of the method of my invention it has been assumed, by way of example, that the large or heavy pieces of material dropped upon the bed are of the same material as that of the bed. While that is usually the case, it is not essential to the method of my invention. When the material to be crushed and ground is highly friable, or of a character to be easily broken by impact, the large or heavy pieces thereof may be reduced in size or weight quite rapidly, tending to reduce the rate of crushing and grinding, particularly the latter. In such cases, the heavy pieces of material under treatment may be supplemented by heavy pieces of a different material, which does not readily break under impact, delivered by the main feed conveyor 11 to the top of the tower, in proper proportion to the heavy pieces of material under treatment. The heavy pieces of such supplementary material may be of mineral formation, such as comparatively hard ore, taconite for example, or may be in the form of heavy pieces or balls of steel or other metal. Within the broader aspects of the method of my invention all, or substantially all, of the heavy pieces of material dropped from the top of the tower may be of material different from the material being crushed and ground, particularly if the latter material is friable and is to be reduced to a fine or substantially powdered condition. In such cases, the crushed and broken material will be returned to the bed in proper amount to maintain the desired depth thereof, and the heavy pieces of the material dropped from the top of the tower will be returned thereto by the main feed conveyor 11. Where the heavy pieces of material dropped from the top of the tower are, in part or in their entirety, the same as the material being crushed and ground, the method of my invention has the additional advantage, previously noted, that they also are broken and reduced in size to desired extent.

It will be understood that changes in detail may be resorted to, both as to the apparatus and the method of my invention, and I intend to include all such variations, as fall within the scope of the appended claims, in this application in which the preferred forms only of my invention have been disclosed.

I claim:

1. In material crushing and grinding means, a tower of substantial height, a shock resistant supporting structure at the bottom of said tower, a travelling bed support resting on said structure free from said tower and adapted to receive and retain a bed of comparatively small pieces of crushed material of predetermined and substantial depth, said bed support being movable relative to said supporting structure for discharge of material from said bed, means for delivering to said support comparatively small pieces of material to be crushed and ground and at a rate effective for maintaining said predetermined depth of said bed, and means for delivering to the top of said tower comparatively large and heavy pieces of material, said tower being unobstructed interiorly for free fall therethrough of said heavy pieces of material for gravity impact with said bed of material effective for subjecting the material of said bed to a high pressure combined crushing and grinding action.

2. In material crushing and grinding means, a tower of substantial height, a shock resistant supporting structure at the bottom of said tower, an endless conveyor resting on and travelling over said supporting structure for discharge of material, said conveyor providing a bed support adapted to receive and retain a bed of comparatively small pieces of crushed material of predetermined and substantial depth, means for receiving crushed material discharged from said conveyor and for separating out therefrom comparatively small pieces of such material, means for delivery to said tower below the top thereof and above said bed support a portion of said comparatively small pieces of crushed material discharged from said conveyor and at a rate effective for maintaining said bed of material at said predetermined depth, and means for delivering to the top of said tower comparatively large and heavy pieces of material, said tower being unobstructed interiorly for free fall therethrough of said heavy pieces of material for gravity impact with said bed of material effective for subjecting the material of said bed to a high pressure combined crushing and grinding action.

3. The method of crushing and grinding materials, said method consisting in forming a bed of comparatively small pieces of material to be crushed and ground and of predetermined and substantial depth, continuously dropping upon said bed from a substantial height free falling comparatively large and heavy pieces of material thereby subjecting said bed to impact by said heavy pieces solely by gravity and efiective for imparting to the material of said bed a combined crushing and grinding action, discharging material from said bed, and delivering to said bed comparatively small pieces of material to be crushed and ground and at a rate effective for maintaining said predetermined depth of said bed.

4. The method of crushing and grinding materials, comprising forming a bed of comparatively small pieces of material to be crushed and ground and of predetermined and substantial depth, continuously dropping upon said bed from a point a substantial height above said bed free falling comparatively large and heavy pieces of material to be crushed and ground thereby subjecting said bed to gravity impact by said heavy pieces effective for imparting to the material of said bed a high pressure combined crushing and grinding action, discharging mate rial from said bed, separating out from such discharged material comparatively small pieces of material and returning the latter to said bed at a rate effective for maintaining said predetermined depth of said bed, and returning to said point for dropping on said bed the larger pieces of material discharged therefrom.

5. The method of crushing and grinding materials, comprising forming a bed of comparatively small pieces of material to be crushed and ground, dropping on said bed from a substantial height free falling comparatively large and heavy pieces of material thereby subjecting the material of said bed to gravity impact by such heavy pieces effective for imparting to the material of said bed a high pressure combined crushing and grinding action, moving said bed at a predetermined rate toward a discharge point and there discharging material from said bed, and delivering to said bed comparatively small pieces of material to be crushed and ground and at a rate effective for maintaining said predetermined depth of said bed.

6. The method of crushing and grinding materials, comprising forming in an enclosed space a bed of comparatively small pieces of crushed material, dropping on said bed from a substantial height free falling comparatively large and heavy pieces of material to be crushed and ground thereby subjecting the material of said bed to gravity impact by such heavy pieces effective for imparting to the material of said bed a high pressure combined crushing and grinding action, moving said bed toward a discharge point and there discharging material from said bed, and delivering to said bed comparatively small pieces of material to be crushed and ground and at a rate effective for maintaining said predetermined depth of said bed.

7. The method of crushing and grinding materials, eom prising forming in an enclosed space a bed of comparatively small pieces of crushed material, dropping on said bed from a substantial height free falling comparatively large and heavy pieces of material to be crushed and ground thereby subj cting the material of said bed to gravity impact by such heavy pieces efiective for imparting to the material of said bed a high pressure combined crushing and grinding action, moving said bed toward a discharge point and there discharging material from said bed, separating out from such discharged material the comparatively small pieces of material and returning the latter to said bed and at a rate effective for maintaining said predetermined depth of said bed, and recirculating with said heavy pieces the larger pieces of material discharged from said bed until said larger pieces have been crushed and reduced to the size of said comparatively small pieces of said bed.

References Cited in the file of this patent UNITED STATES PATENTS Rowland Oct. 18, 1904 Hadsel Apr. 19, 1932 Runyan Aug. 27, 1935 Steele Aug. 31, 1937 Symons Mar. 8, 1938 Bland Apr. 26, 1949 FOREIGN PATENTS Germany Nov. 25, 1901 Great Britain July 19, 1937 

